1. Technical Field
The this disclosure relates to a voltage converting controller and a method of the voltage converting control, particularly to a voltage converting controller and a method of the voltage converting control applied to the constant turn-on time control mode and make the duty cycle of the switch control signal not limited by the minimal off-time.
2. Related Art
The switching voltage converting circuit is one of various the voltage converting circuits. In a switching voltage converting circuit, the power switch is used to modulate the energy stored in an inductive component to supply to the output load, and an input voltage is converted into an output voltage at an output terminal to maintain the fixed output voltage value, and the switching voltage converting circuit provides the output load the required load current. The advantage of the switching voltage converting circuit is that the high conversion efficiency is achieved, thus it can reduce unnecessary heat generation and furthermore reduce the complexity of the heat dissipation design.
FIG. 1 shows a voltage converting controller 100 of the constant-turn-on time control mode in the art. The voltage converting controller 100 is applied to the switching voltage converting circuit 10. The switching voltage converting circuit 10 turns on the high-side power switch 11 and the low-side power switch 12 in a non-overlapping way, respectively; and the switching voltage converting circuit 10 converts the input voltage 14 into an output voltage 15 via the inductor 13 and provides the required load current. The voltage converting controller 100 receives the output voltage 15 or the voltage division of the output voltage 15 via a comparator 110, and compares the received voltage with a reference voltage 120. When the output voltage 15 or the voltage division of the output voltage 15 is less than the reference voltage 120, the comparator 110 sends out a comparison signal to trigger a single-shot circuit 130 to generate a signal of fixed pulse width. And during the period of the foregoing fixed pulse width the driving circuit 140 is used to control the channel of the high-side power switch 11 to be turned on, such that the input voltage 14 supplies the current to the output voltage 15 via the inductor 13. After the end of the foregoing fixed pulse width, the driving circuit 140 controls the channel of the low-side power switch 12 to be turned on.
FIG. 2 is the control time sequence diagram of the switch voltage converting circuit 10 in the steady-state normal operation. The waveform 210 represents the output voltage 15 or its voltage division that the comparator 110 receives. The waveform 220 represents the reference voltage 120. The waveform 230 represents the output signal of the comparator 110. The waveform 240 represents the control signal of the high-side power switch 11. The waveform 250 represents the control signal of the low-side power switch 12. Before the time t1, the channel of the high-side power switch 11 is cut off, while the channel of the low-side power switch 12 is turned on, thus the waveform 210 shows the output voltage 15 or its voltage division decreasing. At the time t1, the voltage value of the waveform 210 starts to be less than the reference voltage 220. Hence, the comparator 110 responds to output the comparison signal, i.e., as described in the waveform 230, and triggers the single-shot circuit 130, thus the driving circuit 140 controls the channel of the high-side power switch 11 to be turned on and the channel of the low-side power switch 12 to be cut off, i.e., as shown in the waveform 240 and the waveform 250. Then the output voltage 15 begins to rise, that is, as shown in the waveform 210. It is noted that in a common comparator 110 applied to the voltage converting controller 100 has the characteristics of the hysteresis. Namely the input comparison level of the comparator 110 determines its output signal, so as to make the output results of the comparator more stable and reduce the unnecessary output jitter noise. This is art for a person having ordinary skill in the art, the details will not be discussed hereinafter.
Furthermore, when the output voltage 15 rises, up to the time t2, the comparator 110 based on its design in the hysteresis region recovers the output signal of the comparator 110. At the time t3, the fixed pulse width of the single-shot circuit 130 is ended, and the driving circuit 140 controls the channel of the high-side power switch 11 to be cut off and the channel of the low-side power switch 12 to be turned on, the output voltage of 15 begins to decline. Until the time t4, the voltage value of the waveform 210 starts again to be less than the reference voltage 220, and therefore it repeats the actions at the time t1.
The advantage of the constant-turn-on time control mode is that the operating clock frequency of the circuit and the load current are a positive correlation. When the load current is light load, the operating clock frequency is lower, reducing the switching loss, thus enhancing the power conversion efficiency. In addition, even if it is designed to be the fixed frequency operation, such as the design that will be described hereinafter, the circuit components of the constant-turn-on time control mode is also relatively simple with larger bandwidth of the loop circuits, which is beneficial for the fast transient responses.
However, when the channel of the high-side power switch 11 is cut off and the channel of the low-side power switch 12 is turned on, at this time due to the transient changes in the current loop it will transmit greater noise interference. The control signal of the power switches are easily influenced, hence, it may cause the control signal to be affected and turned on again shortly after the channel of the high-side power switch 11 starts to be cut off. To avoid this kind of mistaken action, usually the control signal of the high-side power switch 11 has the limitation of the so-called minimal off-time, in order to filter out the transient interference. However, the constant channel turn-on time of the high-side power switch 11, and the limitation of the minimal off-time result in that the duty cycle of the control signal of the high-side power switch 11 has its upper limit, so that within the specification of a rating output voltage 15 the input voltage 14 of the switch voltage converting circuit 10 has the limitation of a minimum value. This restriction is not good for the applications of the use of the batteries to supply the electricity, because the battery voltage lowers down obviously with the process of the electricity supply.
FIG. 3 is another voltage converting controller 300 of the constant-turn-on time control mode in the art. The voltage converting controller 300 is applied to the switch voltage converting circuit 30. The difference between the voltage converting controller 300 and the voltage converting controller 100 as shown in FIG. 1 lies in that between the single-shot circuit 130 and the driving circuit 140, an OR gate 320 is added and the input of the OR gate 320 is connected to the output terminal of the single-shot circuit 130 and the inversion output (provided by the NOT gate 310) of the comparator 110, respectively. The switch voltage converting circuit 30 can break through the upper limitation of the duty cycle of the control signal of the foregoing high-side power switch 11.
FIG. 4A and FIG. 4B are the control time sequence diagrams of the switch voltage converting circuit 30 in the conditions of steady-state normal operation. In the diagrams, the waveforms 410 and 430 represent the received output voltage 15 or its voltage division value of the comparator 110. The waveforms 420 and 440 represent the reference voltage 120. As described in FIG. 4A, when the duty cycle of the control signal of the high-side power switch 11 does not exceed the limitation of its upper limit, the operation of the switch voltage converting circuit 30 is just the same as the switch voltage converting circuit 10. However, when the duty cycle of the control signal of the high-side power switch 11 exceeds the limitation of its upper limit, due to the output voltage 15 or its voltage division value still not being able to be greater than the reference voltage 120 at the end of the fixed pulse width of the single-shot circuit 130, thus the output of the OR gate 320 is determined by the inversion output of the comparator 110 until the output voltage 15 or its voltage division value greater than the reference voltage 120. The output of the OR gate 320 just shifts the state, so that the driving circuit 140 controls the channel of the low-side power switch 12 to be turned on and maintains a span of the “minimum off-time”. However, from FIG. 4A and FIG. 4B we know that in both cases, the value of the output voltage 15 still has a larger gap.